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open a terminal and run
gksudo nautilus
This will open your file browser as root. You should be able to see the contents now.
If the drive still appears empty run
sudo dosfsck -a /dev/sda1 (where sda1 is the drive your are attempting to recover)
This will check the drive for errors. If you find errors you can run
sudo dosfsck -ar /dev/sda1 (use with caution)
This will attempt to repair any problems found

OK. More information is needed
What does fdisk -l show?
Why do you think your registry is corrupted?
What version of windows are you using?
What version of Ubuntu?
Do you have a hard drive enclosure or another computer?

Always nice to do things for zero cost but sometimes that isn't possible. However it may be that the external drive USB controller is faulty and one way to test that is with another enclosure (as Pat says it won't be breaking the bank but first find out if the internal drive is IDE or SATA, or get an enclosure that does both). When it comes to data recovery, which is where this might be going, then that too has free options but often they don't do the job and only a paid program will suffice. What is a fact is that the more you work on this drive to repair it the greater the chance that data on it is wrecked beyond recovery.